Nonrelevant quantum levels limited laser dynamic in narrow-period terahertz quantum cascade lasers

Abstract

The fundamental challenges faced by terahertz quantum cascade lasers are the operations below room temperature, and the recent improved operating temperatures are actually based on the narrow-period designs (2-well resonant-phonon scheme). In this study, we address the critical effect of nonrelevant high-confined quantum levels on the laser threshold current in such designs, i.e. at high temperatures, the resonant-tunneling channels passing through neighboring periods via those levels can dominantly increase the laser threshold current. As a result, the laser dynamics significantly shrink to zero even at 270 K. The channels cannot be easily eliminated as they are derived from the specific configuration of a two-quantum-well periodic structure, in which the width of the period is narrow, and the nonrelevant levels from the lower-well are confined down in energy. It is reasonable to suppress the current flowing across those levels by narrowing the lower-well further, but an appropriate balance of its thickness is essential.